The invention relates to a method for manufacturing a micro-transformer to perform signal transmissions between electrically insulated electric circuits.
In the related art, there are systems for performing signal transmissions between electrically insulated electric circuits so that a dangerous voltage may not pass when a high-voltage such as a surge is applied. One of those systems utilizes an inductive coupling by a transformer (as referred to in JP-A-11-196136 [corresponding to U.S. Pat. No. 6,389,063], for example).
As the recent MEMS (Micro Electro Mechanical Systems) technique improves, the transformer becomes smaller. This makes it possible to integrate the transformer and an integrated circuit. In the following, this small-sized transformer will be called the micro-transformer, and the signal transmission system using the micro-transformer is called the micro-transformer system (as referred to, for example, in JP-B-2001-148277 and in “Key Technologies for System-Integration in the Automotive and Industrial Applications”, IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 20, NO. 3, May, 2005.
FIG. 10 is a sectional view showing a structure of a micro-transformer of the related art. As shown in FIG. 10, the micro-transformer is equipped with a primary coil 7 and a secondary coil 14. These primary coil 7 and secondary coil 14 are separated from each other by an insulating film 23. In order to retain a desired resistance to an electrostatic discharge (ESD: Electro Static Discharge), moreover, the micro-transformer of the related art has to be made such that the thickness of the insulating film 23 is 10 μm or more.
Next, the method for manufacturing the micro-transformer of the related art is described. FIG. 11 to FIG. 13 are sectional views sequentially showing the micro-transformer manufacturing method of the related art. First of all, an impurity-diffused region 2 is selectively formed on a semiconductor substrate 1, as shown in FIG. 11. Next, an insulating film 3 is formed to have a thickness of about 1 μm on the whole face of the substrate by the plasma CVD (Chemical Vapor Deposition) method or the like.
Next, as shown in FIG. 12, the insulating film 3 is partially removed by photolithography or the like to form a first opening 4 and a second opening 5. Then, a metal film is deposited to a thickness of about 3 μm on the whole face of the substrate, and the primary coil 7 is formed by photolithography or the like. At this time, a center pad 8 of the primary coil 7 is made to contact the impurity-diffused region 2 through the first opening 4. There is also formed a pad 9, which is electrically connected with the center pad 8 of the primary coil 7 through the impurity-diffused region 2. Moreover, the pad 9 is made to contact the impurity-diffused region 2 through the second opening 5. Still moreover, the outer-end pad is formed, although not shown, in the outer-end portion of the primary coil 7.
Next, an insulating film is deposited on the primary coil 7 by the plasma CVD method or the like, as shown in FIG. 13. Then, the surface of the insulating film is flattened to form the insulating film 23 having a thickness of 10 μm. Next, a portion of the insulating film 23 is removed by photolithography or the like to form an opening 24. Thus, the pad 9 is exposed at the opening 24 to the outside. Although not shown, moreover, the outer-end pad is exposed to the outside at the opening other than the region shown in FIG. 13.
Next, as shown in FIG. 10, a metal film is deposited on the insulating film 23, and the secondary coil 14 is formed by photolithography or the like. The shape of the secondary coil 14 is made substantially identical to that of the primary coil 7. In the secondary coil 14, a center pad 15 is formed at the center portion, and an outer-end pad 16 is formed at the outer-end portion. Simultaneously as the secondary coil 14 is formed, the opening 24 is covered with the metal film. In this case, the metal film covering the opening 24 contacts the pad 9, and becomes the electrode pad of the center-side end portion of the primary coil 7. Therefore, the signal from the primary coil 7 is accepted by the secondary coil 14 so that it can be transmitted to the outside from the center pad 15 of the secondary coil 14 and the outer-end pad 16 of the secondary coil.
However, the aforementioned manufacturing method of the related art is troubled by lower throughput because the insulating film 23 formed on the primary coil 7 must be thick. The large thickness of the insulating film 23 makes the insulating film 23 vulnerable to being cracked by stress. When the insulating film 23 is partially removed by etching, the throughput is lowered. When this etching is performed, a mask member having a high selection ratio must be used.
In view of the above, it would be preferable to provide a micro-transformer manufacturing method capable of improving the throughput. It would further be preferable to provide a micro-transformer manufacturing method capable of preventing the insulating film between the coils from being cracked. It would even further be preferable to provide a manufacturing method capable of manufacturing the micro-transformer without using a mask member having a high selection ratio.